Electrolytic recording medium



United States Patent Ofiice 3,122,489 Patented Feb. 25, 1964 3,122,489 ELEQTEEBLYTZHC RECGRDWG MEDIUM Gerald Mandel, New Rochelle, N32,, and Arthur W. Sperling, Emerson, N1, assignors to Hogan Faxiinile Corporation, New York, N No Brewing. Filed Ilec. '7, I959, Ser. No. 37,477 7 Claims. {61. 204-2) This invention concerns electrolytic recording and in particular an improved electrolytic recording medium used to transform elec.rical pulses into graphic recordings.

Electrolytic recording is the art of recording by means of the electrolytic action of an electric current upon a properly sensitized medium. The recording current is localized to produce marks under control of a machine or operator. In facsinn'le recording it is common practice to scan line-by-line a record sheet with a stylus which carries current modulated according to the instantaneous density of the subject to be reproduced. A

roperly sensitized recording medium, such as an impregnated pager, -nay be passed under a reciprocating stylus carrying the recording current, or the recording operation may be carried out in a number of Ways well known to those skilled in the art. In order to produce a mark or image on the recording paper, it must carry on its surface or impregnated in its structure marking compounds which change color or optical density, or both, in response to the passage of the electric current.

Heretofore, electrolytic recording papers have been made both for use with electrodes of the eroding and non-eroding electrode types. In the recording process using the eroding type electrode or anode it has been necessary for the anode to supply metal ions to combine with marking compounds impregnated into the paper to form colored marks. In marking this type of recording paper the metal anode is gradually consumed. This consumption or erosion of the anode has proved disadvantageous since frequent replacements thereof are required. In the recording process using the non-eroding type electrode, anodic erosion thereof can be avoided thus eliminating the need for replacement. However, in those cases in which electrolytic recording has been accomplished Without the erosion of the anode as in the marking of azo papers, the results have not been generally successful commercially because of lack of stability of the paper. In order for an electrolytic recording paper to be considered stable it should be storable for several months after impregnation, and for several months after recording, and remain substantially unchanged in the color of he background and in the color of the recorded mark.

It has been discovered that excellent marks are produced in a recording medium marked between cathode and anode electrodes by incorporating a metal-bearing compound in the paper inipregnant. The metal-bearing compound reacts with the hydroxide ions (OI?) formed at the cathode to form a colored mark. No erosion of the anode occurs when a platinum anode is used. Further, it has been discovered that an improved recording medium is produced by adding to the metal hearing compound a polyhydroxyphenolic compound such as gallic acid, catechol, pyrogallol, hydroquinone, gentisic acid, and others known in the art. However, a metal impregnated recording medium containing any of these polyhydroxyphenolic compounds has been found to have a short storage or shelf life. In a few days, the impregnated recording paper discolors prior to being marked even though stored in hermetically sealed containers. Subsequent to marking the background of the marked paper darkens appreciably in a few days or weeks, to ob some or almost obscure the recorded subject matter.

In the past many attempts have been made to increase the shelf-life of the recording medium such as by storing it in hermetically sealed containers with inert gases, and by adding vmious known stabilizers to the impregnant of the recording medium. Stabilizers which have been used with only limited success are thiourea, hydrazine and salts thereof, hydroxylamine and salts thereof, and aniline and salts thereof. Some of these so called stabilizers act wholly or partially as oxidation inhibitors preventing degradation of the polyhydroxyphenolic compound in the impregnant. In any event, the stabilizers employed heretofore have improved the storage or shelf-life of marked or unmarked recording paper only for a limited time. The need for an electrolytic recording medium having a storage or shelf-life of a year or more has not been met by any of the media using a non-eroding electrode.

An object of the invention is to provide an improved electrolytic recording paper adapted to be used with noneroding electrodes and which has improved shelf-life.

Another object of the invention is to provide an im proved electrolytic recording paper adapted to be marked upon the passage of an electric current therethrough be tween non-eroding electrodes and in which the background does not become discolored Within a year or more.

It has been discovered according to the present invention that an improved electrolytic recording mediurn is provided by incorporating a metal salt with a polyhydroxyphenolic compound in an electrolyte with a metal bisulfite. Upon the passage of an electric current through the medium a colored mark is formed. The metal com pound is believed to react with hydroxide ions formed at the cathode by the current, and in the presence of an oxidizing agent, such as oxygen conveniently supplied from the ambient air, an intermediate higher bydroxide or hydrated oxide of the metal salt is formed. The polyhydroxyphenolic compound reacts with the intermediate compound to yield an intensely colored mark. In the event the metal bisulfite is omitted a degradation reaction takes place during storage of the paper resulting in visible discoloration thereof within a few days.

In the event one of the stabilizing agents generally employed in electrolytic recording paper of the type used With an eroding electrode are incorporated in the formulation of the present invention instead of metal bisulfite visible discoloration is prevented for a few weeks. Such stabilizers are thiourea, alkyl derivatives of Lhiourea, dithiobiuret, hydraz ne and salts thereof, and hydroxylamine and salts thereof. The stabilization effect obtained by these agents is due to their counteraction of a small amount of potential free radical degradation. Inasmuch as discoloration proceeds after a few weeks there is evidently a further degradation action which ta es place.

It has been found that a recording paper made as taught herein has a shelf ife of at least a year. The additional shelf life is believed due to the action of the metal bisulfite in inhibiting degradation.

The mechanism by which the stabilizers such as thiourea act is believed to be by providing a termination step for a free radical degradation chain reaction as:

H2NCN]1z OH ll Semiquinone thiourca radical produced in degradation -01: H2NCNH2 OH- OH polyhydroxyfree radical phenolic It is known that metal bisulfites are not oxidized through a free radical mechanism.

Formulations to impregnate about 30 square feet of white porous paper are set out hereinafter. The formulations when impregnated into about 30 square feet of white porous paper, will be electrolytically markable upon passage of an electric current therethrough from electrodes applied to the paper. The electrodes need not be consumed in the marking process, and no supplementary heat is required to intensify the mark. The paper will dry out in air after marking, or drying may be accelerated by subjecting it to an air stream or a low-temperature heater bar.

Example 1 Grams Manganese chloride, MnCl .4H O l Gallic acid, C H O .H O 1 Sodium chloride, NaCl 10 Sodium bisulfite, Nail-I50 1.0 Sodium hydroxide, 5 N NaOI-I 1.0

Water 100 A paper impregnated as in Example 1 was found to withstand storage for five to six months without discernible discoloration in both market and unmarked states.

Example 2 For comparative purposes a formulation was made omitting sodium bisulfite.

Grams Manganese chloride 1.0 Gallic acid 1.0 Sodium chloride 10.0 Sodium hydroxide 1.0

Water 100.0

A marked paper impregnated as in Example 2 was found to have discolored after less than 14 days to the extent that the mark was hardly discernible from the background. An unmarked paper so impregnated was found to be similarly discolored after the same period.

Example 3 For comparative purposes a formulation was made using thiourea instead of sodium bisulfite.

d A marked paper impregnated as in Example 3 was found to have discolored after less than 2 months to the extent that the mark was hardly discernible from the background. An unmarked paper so impregnated was found to be similarly discolored after the same period.

Example 4 A formulation was made using both thiourea and sodium bisulfite.

Grams Manganese chloride 1.0 Gallic acid 1.0 Sodium chloride- 10.0 Thiourea 2.5 Sodium bisulfite 1.0 Sodium hydroxide 1.0

Water 100.0

Paper impregnated as in Example 4 was found to Withstand storage for over a year without discernible discoloration in both marked and unmarked states.

It is therefore concluded that although there is a tendency toward some free radical degradation of the formulation in accordance with the invention the majority of the degradation proceeds through a mechanism other than the free radical mechanism. Consequently the preferred formulation includes both an agent for terminating ree radical degradation and a metal bisulfite.

Paper impregnated as in Example 5 Was found to with stand storage as well as that made from the formulation of Example 4. In addition because of the use of the anti-fringing and pH buifering agents somewhat improved resolution was had.

Sodium chloride is used to provide an electrotlyte. Other electrolytes may be used alternatively, such as sodium and potassium nitrates and equivalent electrolytes Well known in the art.

In the above examples, the quantities given for sodium chloride may vary from zero to 30 grams (the solubility limit) depending upon the degree of electrical conductivity desired. Similarly, if sodium nitrate is used for an electrolyte, its concentration may vary from zero to grams (the solubility limit).

In any of the above formulations, instead of gallic acid, there may be substituted catechol, pyrogallol, or other suitable polyhydroxyphenolic compounds, such as, but not limited to, hydroquinone and gentistic acid. Gallic acid is preferred because of the greater intensity of the mark with low marking currents. The quantities given for the gallic acid represent the approximate solubility limit for this material. If less than this amount is used the intensity of the resulting mark is decreased.

The metallic compounds which may be used are those of metals having a stable or stabilizable higher oxidation 7 state in solution and which are capable of being oxidized to the higher oxidation state in the presence of base. Such compounds include but are not limited to Mu C0 Ni Cr Ag Sn Pb 'Il V The quantities of the nickelous, cobaltous and manganous salts or combinations thereof used may be varied up to their respective solubility limits.

Citric acid is employed in the formulation as an antifringing agent and as a pH bufier to add stability.

Thiourea is employed to aid stability but it is relatively ineffective in the absence of bisulfite as stated before.

Sodium phosphate is employed as an antifringing agent and as a pH buffer.

The metal bisulfites which may be used may be chosen from the class including sodium bisulfite, potassium bisulfite, lithium bisulfite, other alkali metal bisulfites, sodium metabisulfite, and other alkali metal metabisulfites.

Although specific amounts of the ingredients of impregnant are set out in the examples, variations in the quantities of ingredients also will be effective. For example, While 10 grams of sodium chloride are preferred any amount providing adequate conductivity may be used as from approximately 3.0 grams to 30.0 grams. The amount of gallic acid used is determined by the quantity necessary to provide a satisfactory mark and may be varied between 0.5 gram to 2.0 grams. Likewise, the manganese chloride may be varied from 0.5 gram to 5.0 grams. Variations in the amount of citric acid and of monosodium phosphate used will cause a variation in the pH and consequently in the sensitivity of the paper to the marking current. The citric acid may be varied up to 3.0 grams, and the monosodium phosphate may be varied up to 1.0 gram. The sodium bisulfite, NaHSO may be varied up to 20.0 grams. Sodium metabisulfite, Na S O may be substituted for the sodium bisulfite or mixtures therewith may be used. Potassium, lithium or other alkali metal bisulfites may be alternatively used. Thiourea, like the bisulfite, may be varied up to its solubility limit of 9.0 grams.

Enough sodium hydroxide is used to produce a final pH of the impregnant solution between 2.5 and 8.5, preferably about 5.0. At lower pH levels the solubility of the polyhydroxyphenolics is depressed while at higher pH values the polyhydroxyphenolics are unstable towards air oxidation. The small concentration of GH ions produced by the sodium hydroxide has been found insufficient to cause oxidation of the metal ions and darkening of the paper.

The impregnated paper will be electrically markable upon passage of an electric current therethrough from electrodes applied to the paper. The electrodes need not be consumed in the marking process, and no supplementary heat is required to intensify the mark. The paper may be dried in air after marking, or drying may be accelerated by subjecting to an air stream or a low-temperature heater.

The marked paper will have a storage or shelf-life without material discoloration or degradation of a year or more. Recording paper stabilized with sodium bisu phite and subjected to extensive accelerated aging tests under strong light and in the presence of oxygen and other oxidants exhibits remarkable and unexpected stability and resistance to discoloration in both marked and unmarked states.

It is to be understood that, while the term paper has been used in this specification, the marking compounds are equally applicable to fabrics or any material of the same general nature which may be suitable for use in electrolytic recording processes. Accordingly, the term electrolytic recording medium is to be construed as defining any or all of such materials.

While the invent-ion has been described with reference to specific embodiments thereof, it will be understood that 6 other embodiments may be resorted to without departing from the invention. Therefore, the form of the invention set out above should be considered as illustrative and not as limiting the score of the following claims.

We claim:

1. In an electrolytic recording medium of the type wherein a white porous support is impregnated with an aqueous solution containing a metal compound including a metal selected from the group consisting of Mu C0 Ni Cr Ag Su Pb Tl V and a polyhydroxyphenolic compound selected from the group consisting of catechol, pyrogallol, gallic acid, hydroquinone and gentisic acid, the combination therewith of a two component stabilizer, one component of said stabilizer selected from the group consisting of thiourea, alkyl derivatives of thiourea, and dithiobiuret and the other component of said stabilizer selected from the group consisting of water soluble alkali metal bisulfites and meta-bisulfites.

2. An electrolytic recording medium according to claim 1 in which the metal compound is a manganous salt.

3. An electrolytic recording medium according to claim 1 in which the metal compound is a manganous salt and the polyhydroxyphenolic compound is gailic acid.

4. An electrolytic recording medium according to claim 1 in which the metal bisulfite is sodium bisulfite.

5. An electrolytic recording medium according to claim 1 in which the alkali metal meta bisulfite is sodium metabisulfite.

6. An electrolytic recording medium comprising a substantially white porous support impregnated with a solution comprising in parts by relative weight manganese chloride 0.5 to 5.0 grams, gallic acid 0.5 to 2.0 grams, sodium chloride 3.0 to 30.0 grams, citric acid 1.0 to 3.0 grams, sodium phosphate 0.4 to 1.0 gram, thiourea 2.5 to 9.0 grams, and sodium bisulfite 1.0 to 20.0 grams, and water grams.

7. An electrolytic recording medium comprising a substantially white porous support impregnated with a solution comprising manganese chloride, gallic acid, sodium chloride, citric acid, sodium phosphate, thiourea, and sodium bisulfite, and Water.

References Cited in the file of this patent UNITED STATES PATENTS 168,466 Edison Oct. 5, 1875 863,761 Schmid Aug. 20, 1907 1,343,978 Little June 22, 1920 1,918,492 Schmidt July 18, 1933 2,082,735 Heinecke June 1, 1937 2,306,471 Solomon Dec. 29, 1942 2,339,267 Hogan Jan. 18, 1944 2,358,839 Wagner Sept. 26, 1944 2,430,254 Solomon Nov. 4, 1947 2,440,526 Solomon Apr. 27, 1948 2,570,096 Calkin Oct. 2, 1951 2,663,656 Miller et al. Dec. 22, 1953 2,776,252 Schwartz Ian. 1, 1957 2,949,409 Richards Aug. 16, 1960 3,010,883 Johnson et al. Nov. 28, 1961 OTHER REFERENCES Bureau of Standards Circular C426, Inks, pages 32 and 33, October 16, 1940.

Ilford Manual of Photography, pages 400 and 401, May 1958. 

1. IN AN ELECTROLYTIC RECORDING MEDIUM OF THE TYPE WHEREIN A WHITE POROUS SUPPORT IS IMPREGNATED WITH AN AQUEOUS SOLUTION CONTAINING A METAL COMPOUND INCLUDING A METAL SELECTED FROM THE GROUP CONSISTING OF MNII, COII, NIII, CRIII, AGI, SNII, PBII, TII, VIII, AND A POLYHYDROXYPHENOLIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF CATECHOL, PYROGALLOL, GALLIC ACID, HYDROQUINONE AND GENTISIC ACID, THE COMBINATION THEREWITH OF A TWO COMPONENT STABILIZER, ONE COMPONENT OF SAID STABILIZER SELECTED FROM THE GROUP CONSISTING OF THIOUREA, ALKY, DERIVATIVES OF THIOUREA, AND DITHIOBIURET AND THE OTHER COMPONENT OF SAID STABILIZER SELECTED FROM THE GROUP CONSISTING OF WATER SOLUBLE ALKALI METAL BISULFITES AND META-BISULFITES. 